offer function-level control over tracing (#5194)

* wiggle: fix compilation with async functions when tracing is off

Fixes #5202

* switch tracing config from a boolean to a struct

This will enable more complex tracing rules in the future

* rename AsyncConfField to FunctionField

It is going to be reused for cases other than just async functions

* add support for disabling tracing per-function

This adds a `disable_for` syntax after the `tracing` boolean.  For
example:

```
wiggle::from_witx!(
    tracing: true disable_for {
        module1::foo,
        module2::{bar, baz},
    }
)
```

crates/wiggle/test-helpers/examples/tracing.rs

@@ -15,6 +15,9 @@ pub enum RichError {
 // Define an errno with variants corresponding to RichError. Use it in a
 // trivial function.
 wiggle::from_witx!({
+    tracing: true disable_for {
+        one_error_conversion::foo,
+    },
 witx_literal: "
 (typename $errno (enum (@witx tag u8) $ok $invalid_arg $picket_line))
 (typename $s (record (field $f1 (@witx usize)) (field $f2 (@witx pointer u8))))